The amount of expendable hardware required to place a satellite in orbit is the primary cost driver for access to space. Specific impulse (Isp) is one of the major factors determining the size of the vehicle required to accelerate a spacecraft to orbital velocity. The specific impulse of rocket propulsion is typically limited to approximately 450 seconds. Air breathing hypersonic propulsion systems, such as scramjets, have much higher specific impulses and have been proposed for space transportation, but have proven to be impractical due to their limited thrust potential.
Because scramjet propulsion has low thrust and must operate within the atmosphere, gravity and drag losses are high. Under these conditions, rocket propulsion is more efficient. Electric propulsion has a higher Isp (3,000 seconds or more), but has considerably lower thrust. As a result, electric propulsion cannot be directly employed as an ascent propulsion system.
Rotorvators have been proposed to propel a payload into a higher orbit. The rotorvator tether system is placed in an elliptical orbit and its rotation is timed so that the tether is oriented vertically below the central facility and swinging backwards when the system reaches perigee. At that point, a grapple mechanism located at the tether tip can rendezvous with and capture the payload, which is moving in either a lower orbital trajectory or a suborbital trajectory. Half a tether rotation later, the tether releases the payload, tossing it into a higher energy orbit.
Rotorvators require the tether to rotate about a massive central body. The purpose of the tether in a rotorvator concept is to transmit the centripetal force that keeps the payload in its circular motion around the rotorvator. As such, the tether does not bend. The acceleration of the target using a rotavator depends on the rotational rate of the tether and the mass of the central body.
However, rotorvator systems are complex and difficult to implement. Accordingly, an improved tether-based system may be beneficial.